Dry eye disease (DED) is a chronic immune-mediated disorder of the ocular surface, characterized by disruption of the epithelial barrier and sustained ocular surface inflammation, which in severe cases results in blindness from corneal ulceration and scarring. DED is arguably the most common ophthalmologic condition for which patients visit eye care professionals, with more than 16 million US adults estimated to have been diagnosed with DED (several folds higher than age-related macular degeneration). Notably, the prevalence of DED increases with age. The financial burden of DED to the US healthcare system is vast, with direct costs and expenses from productivity losses estimated to exceed $3.5 billion annually. The work of several laboratories, including the Principal Investigator's, has revealed key insights concerning the immunopathogenesis of DED. There is now strong evidence that T helper-17 (Th17) cells are critical promoters of immune-mediated damage to the ocular surface. The PI's laboratory has shown that memory Th17 (mTh17) cells maintain disease chronicity, resulting in ocular surface epitheliopathy that persists for many months after exposure to desiccating stress. Moreover, data from the PI's lab show that regulatory T cells (Tregs) are defective in suppressing the Th17 response in DED. In addition, the PI's lab has identified critical mechanisms controlling antigen-presenting cell (APC) activation and trafficking in DED, as well as effector Th17 priming, expansion and homing. Despite substantial progress in unraveling underlying immunopathogenic mechanisms, important questions remain unanswered. We hypothesize that (1) chronic DED is characterized by long-term impairment in the function of regulatory T cells (Tregs) that normally maintain immune quiescence, and that (2) the aged are more susceptible to DED due to a larger pool of memory Th17, whose function is inadequately regulated by aged Tregs. The principal objectives of this project are to (i) precisely define the phenotypic changes that characterize, and the mechanisms that promote long-term Treg dysfunction in DED; (ii) determine the cellular precursors of mTh17 cells, as well as the cytokine mechanisms that influence the generation of immunologic memory; and (iii) define the factors that amplify mTh17 immunity in aging. To achieve these objectives, three specific aims have been defined to answer the following questions: Aim 1: What are the causative mechanisms of enduring Treg dysfunction in DED? Aim 2: Which effector Th17 subsets are predisposed to enter the memory pool (thus permitting DED chronicity), and what factors regulate this process? And finally Aim 3: What are the cellular and molecular mechanisms that augment Th17-mediated DED in aging? It is anticipated that this research will have significant translational impact given the high prevalence of DED, the still limited knowledge we have regarding its immunopathogenesis, and the relative scarcity of effective treatments.